The Her2/neu/ErbB2 gene encodes a member of the epidermal growth factor (EGF) receptor family of receptor tyrosine kinases. This protein has no ligand binding domain of its own and therefore cannot bind growth factors. However, it does bind tightly to other ligand-bound EGF receptor family members to form a heterodimer, stabilizing ligand binding and enhancing kinase-mediated activation of downstream signaling pathways, such as those involving mitogen-activated protein kinase and phosphatidylinositol-3 kinase. The Her2/Neu (c-erbB-2) proto-oncogene is a transmembrane receptor tyrosine kinase that is clinically indicated in a number of carcinomas, including breast and ovarian tumors. Overexpression of the c-erbB-2 protein has been associated with ductal breast cancer, as well as pulmonary and gastric adenocarcinomas. A correlation between Her2 and p53 has also been documented, as overexpression of both proteins has been associated with early invasion and metastasis in bladder cancer.